Nowadays, it is known the use of a field of steerable mirrors, or heliostats, that concentrate the solar radiation in a receptacle which occupies the upper part of a tower, erected to the south of said field. There exist precedents for this type of configuration, both at the experimental constructive level and at the patent level, among which we can mention, due to its antiquity, the U.S. Pat. No. 4,117,682, which presents a sector-divided field of heliostats, having a tower with a central receptor in each sector of the field.
Alternatively, there may be tower arrangements, but using the whole building facade, not only an upper receptacle, as it can be seen in DE 10248064 A1. Prior to this patent, we can mention U.S. Pat. No. 4,136,674 in which the external surface of the central tower where the radiation reflected by the mirrors is received, is constituted by a bundle of tubes covering it completely. Similarly, we can mention the European Patent Application EP 0106688 A2, intended to improve the performance of tube collectors when they are designed to produce steam, so that it is sought an appropriate distribution of the thermal load between the tubes designed for the boiler, where a phase change is produced, and for the superheater, where there is a temperature increase of the dry steam already separated from the liquid phase, which is recirculated.
In these two patents, two especially serious problems of this panel or collector arrangement or configuration are evident: as these tubes are bare, the natural convection of the circulating air extracts a lot of heat from the irradiated surface, thus decreasing the effectively collected heat, that is, transferred to the heat-conducting fluid, with which the performance of the collector is low or very low; and besides, the oxygen of the air may quickly oxidize the external surface of the hot tube, which will have to be painted with a substance having a high solar absorptivity and a low emissivity, properties which are degraded when the oxygen chemically reacts with said especial paint layer.
The standard answer to these problems is enclosing the radiation collecting surface in a receptacle or a box where a considerable vacuum has been made, and its closing, at the part where the radiation strikes, will be a transparent cover, generally made of glass, in order to let the radiation go through to the interior of the box. This arrangement is even followed in low- and medium-temperature collectors, functioning with direct radiation (not reflected with concentration), though in this case the temperatures reached are much lower. In any case, this arrangement enables to minimize the level of convective heat losses, since there are no convection currents inside the vacuum collector receptacle, and the chemical aggression of oxygen against the external layer of the radiation absorbent surface is also avoided.
This solution, however, has a problem, namely, that the glass of the transparent cover has to work with pressure differences between its walls of approximately one atmosphere, that is, about 100,000 pascals. This pressure difference is very high for a material having a very low elastic limit, such as glass (and it would be even worse for some transparent plastics, because they would also have to withstand temperatures much higher than room temperature, though not as high as the ones of the radiation absorbent surface). This means that the dimensions of the glass cover (or the cover in general), will be very limited by the internal mechanical stresses induced by the aforementioned pressure difference. As in the case of a uniformly loaded beam (that in our case corresponds to a uniform pressure difference) the maximum bending moment depends on the square of the length of the beam, this length will be considerably limited by the effect of the difference between the external pressure (the local atmospheric pressure) and the internal, practically non-existent, one, though in the constructive reality a vacuum of one thousandth of an atmosphere is considered to be sufficient. The glass cover could be very thick, to increase the transverse moment of inertia, thus decreasing the internal stress needed for balancing the induced bending moment, but this possibility presents, in turn, another inconvenience, since there is not any absolutely transparent glass, and the thicker the glass is, the greater the amount of absorbed radiation within itself there would be, which generates higher internal temperatures, and a greater gradient for it, because it is only refrigerated through the external wall, which generates the appearance of mechanical stress induced by thermal differences, that may also be larger than the ones produced by the external and internal pressure difference.
Along with the problems highlighted, another equally important problem should be noted, namely, that the expansions produced when the collector elements go from room temperature to the operation one, which may imply an increase of 500° C. or equal Kelvin in the SI system. In view of the fact that the coefficient of linear expansion of conventional steel is of about 10 millionths per ° C. the increases in length are of about 0.5%, which is quite considerable and has to be taken into account in the design of the radiation absorbent surface, and in the matrix or body on which it is supported, inside which the heat-conducting fluid will flow, to extract the heat and carry it to the main thermodynamic focus. When discussing expansions, it should also be noted that these are smaller on the glass cover than on the absorbent surface and its supporting body. In fact, even the other walls of the collector receptacle, different from the transparent cover, will also be at temperatures much lower than the radiation absorbent parts, otherwise a large part of the energy would escape through these walls, by conduction and convection.
The case described above forms the entire group of problems to be solved by an invention using the natural physical mechanisms, along with the artificial ones that are required, in order to be able to absorb an important fraction of the solar radiation thermal energy, when it is concentrated by means of convergence, on a collector, of the radiation reflected by an array of mirrors whose total reflection surface is much higher than the radiation absorbent surface, contained inside the collector receptacle. A system that could achieve the absorption of thermal solar energy by a heat-conducting fluid avoiding the existing inconveniences of prior art systems was therefore desirable.